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Hydroponic Ventilation Fans Cooling CO2

Temperature of Air in Grow Room

Plants will germinate and grow better and more rapidly within a suitable temperature range. Although they will tolerate temperatures between 60°F and 92°F (17°C – 33°C) these extremes are at the limits of their range. The ideal temperature range is 70°F – 83°F (21.1°C – 28.3°C) while using CO2 you might want to be in the higher temperature range but during darkness times a lower temperature is best. Seedlings particularly will benefit from a heating mat beneath them to encourage strong root growth. Try to stick around the 72°F to 77°F (22.2°C to 25°C) range for all growth.


Planning air movement

Your plants use both light and CO2 to photosynthesise. Because of this, and also because of the heat build up from powerful lamps, your grow room needs to be well ventilated. Both intake and exit vents are essential and will bring CO2 rich air into the room whilst expelling CO2 depleted air. Ideally you will use an extraction fan high up in the room to facilitate air flow and prevent stratification. Fans are rated by the volume of air they move and are measured in either cubic feet per minute (CFM) or cubic metres per hour (M3/H). Calculate the size of your room by multiplying the width by the height by the depth and buy a fan that will change the air once every 1-5 minutes.It is far more efficient for a fan to pull air than push air. Consequently the extraction fan should pull enough air through the inlet vent to sufficiently ventilate the room. If for some reason the supply of fresh air through the inlet vent is compromised you may want to consider using a fan to pull air into the room as well. However this is seldom necessary.

Oscillating fans

Using an oscillating fan within the grow room will help with air circulation. This is particularly important during flowering when the buds are densely packed and susceptible to mould. A gentle breeze ruffling your plants will also help strengthen the stems. In the wild, whenever a plant bends in the wind it creates tiny fissures in the stems which the plant then repairs with scar tissue. This is how stems become strong and woody. Without this movement your marijuana plants will be more fragile and have weaker stems.


If you have to move air a distance to an exterior vent, or you have a large room with fans a long way from a wall, you may need to use ducting. Ducting comes in standard sizes that will fit all of your fans and filters. Flexible ducting is the easiest to work with, but it is important to try and keep it in straight lengths. Avoid having the ducting turning too many corners as this will seriously compromise the efficiency of your ventilation system.


Fresh air has on average 390 PPM (parts per million) of CO2. Marijuana plants can effectively synthesise 1500 – 2000 PPM. Many growers augment the CO2 supply to their plants and this can dramatically increase yield. It does require a bit more expense but is considered well worth it. Don’t worry though if you can’t afford it as your plants will still produce without it.If you do decide to use CO2, by far the most effective way is to buy a CO2 bottle and a kit from a hydroponics store which will allow you to regulate flow though a timer. Run the CO2 through rubber tubing, pierced with fine holes and arranged around the tops of your plants (CO2 is heavier than air). Remember to set your timers so that CO2 comes on when the fans go off.There are cheaper, though less efficient, ways to create CO2 and these include; as a by-product of fermentation, using dry ice, mixing vinegar and baking soda and even spraying the leaves with soda water. Avoid using the fumes of a propane heater as these contain other noxious gasses and often represent a fire risk.

20 thoughts on “Hydroponic Ventilation Fans Cooling CO2

  1. carrie says:

    What is ibest for mold

  2. Neil says:

    Very helpful and informative got lots of thing I was doing wrong, due to lack of knowlage thanks so much for the help Neil

  3. Big bud says:

    Iv just potted my two week cuttings they have been under light now for the first 18hours of there life they have been fed but there leaves are a little droopy and soft what could be wrong ?

  4. dang ol stoner says:

    I’ve got myself a CO2 tank I use for making club soda and draft beer.
    So I could just mist my plants with soda water?!!

  5. Rob says:

    I thought this stuff was easy

  6. That1guy says:

    metal halide and hps lights. mh for veg hps for veg… led’s expensivet o get… but cheaper on electric bill.. they are effective but hps i think is better than led. yes ballast. highley recomend a fan… air flow helps prevent bug disease and gives ur plant exercise. nutrients its opinion. fora beginer… i recomend getting a 3 pack…. micro nutes… veg nutes… flowering nutes. 20-20-20 sounds like miracel grow and not right for cannabis.. if you use fox farms line.. use sparringly at first…. it weill burn ur plants.

  7. tulip says:

    hey thom, you seam like you now what ur doing i was wondering if you could help me im a beginer grower and here is loads of info online but lots of people are saying different things, what lights do u use, and do i really need a ballast and extractor fan? i was thinking of just opening the window? and what plant food do you use for each stage? oh and LED lights? whats ur opinion tanx

  8. rick says:

    my tempuratures in my grow box is about 110F and 48C i have 3 cfl 40 watts ech so thats 120 and i have a ex fan up top what do i do.

    Admin – http://forum.howtogrowmarijuana.com/grow-room-design-setup/

  9. Thom says:

    Yes you will light. the plants will need direct light for at least 16-24 hours during the veg and 12 hours during flower. I set up a grow box made from a shelving unit 2’x4′ used cfl and computer fans. I get about 1/4lb each month with a rotating perpetual harvest.

  10. NORBERTO COSTA says:

    i just started growing marijuana indoors, im trying to figure out the cheapest ; most effeicent way to grow my plants w/out all the above expensive products being its my first time i wanna make sure im doing it correctly before investing money into it,i currently have 6 plants in my room which happens to be my bedroom; there is a sliding glass window in the room that gets about 5 hours of direct sunlight im wondering having my plants near the windows getting natural sunlight if i should still invest in some kinda of lighting for them,also if you can show me the easiest; best way to determine the sex,also currently im using a 20-20-20 water soulube fertilizer which i read was the best ratio for the vegatative state & then to use either phospurous alone or a fertilizer with a high ratio of phospurous,please let me know if im on my way to getting some decent bud to smoke,with the conditions that i have my plants growing at thank you hope to hear from you soon.

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